The print head of the flat bed printer may be mounted on a gantry above the flatbed surface. In case of a gantry, the gantry may be moving over the flat bed surface in a first direction, while the print head is movable along the gantry in a second direction perpendicular to the first direction. In case of a print head having a same width as the flat bed surface, the print head may be moving in one direction over the flat bed surface. The control unit is connected to the print head for controlling the print head. The control unit is also controlling the movement of the print head over the flat bed surface. The distance from the print head to the flat bed surface may be variable in order to allow the printing of 3D images.
The recording material may be hot melt ink or a UV curable ink. When the latter ink is used, the print head is also provided with UV lamps for curing the recording material when ejected on the media object placed on the flatbed surface. The flatbed surface usually has the form of a rectangle, for example of a width of 1.22 m and a length of 1.22 m, or of a width of 1.22 m and a length of 2.44 m.
A media object with dimensions in the plane of the flat bed surface smaller or equal to the dimensions of the flat bed surface can be placed on the flat bed surface. The media object has a height in a direction perpendicular to the flat bed surface. Flat bed print systems usually apply recording material, like colorants, on a media object placed on the flat bed in the form of toner or ink according to a digitally defined, two-dimensional pattern of pixels with values that indicate a composition of these colorants. This pattern is generated out of a target digital image, that may comprise objects in either vectorized or rasterized format, using conventional techniques like interpretation, rendering, and screening by a raster image processor. The processing of a target digital image includes colour management to convert colour values of the pixels in the target digital image into composition values related to the printer colour space as is set up by the colorants of the print system. Depending on the intended print quality and the characteristics of the print process, the pixels of the pattern may be printed in more than one pass, wherein a position of the media object on the flat bed has an opportunity to receive a colorant in one or more of the passes of the print head across the flat bed.
The printer also comprises a camera for capturing digital camera images from the flat bed surface. The camera is positioned above the flat bed surface. The reach of the camera includes at least part of the flat bed surface, but preferably the whole flat bed surface. If a media object has been placed on the flat bed and the camera captures a digital camera image, an image processing component of the control unit is configured to derive camera coordinates of the media object in the captured digital camera image.
An operator may place a media object on the flatbed surface in order to print a digital target image on the media object. The digital target image has been offered to the control unit. The media object may be a paper sheet, a plastic sheet, canvas, a glass plate, a plastic plate, a wooden board, a plastic board, a metallic board, a wooden panel, a plastic panel, a metallic panel or a sheet, plate, panel or board of a different kind of receiving media.
The flatbed printer comprises a camera system connected to the control unit and positioned above the flatbed surface. The camera system comprises a camera above the flat bed surface at a predetermined height from the flat bed surface. The camera is calibrated to take digital camera images of the flat bed surface. Each location in a digital camera image taken by the camera corresponds to a position in the plane of the flat bed surface. A non-linear calculation executed by the image processing component in the control unit couples a pixel in the digital camera image to a position of the flat bed surface. The image processing component is configured to analyse the digital camera image of the flat bed surface in order to detect a media object placed on the flat bed surface.
According to the prior art actual coordinates of points on the media object on the flat bed surface corresponding to points of the media object in the digital camera image are calculated by the control unit. Then another calculation derives print head coordinates from the actual coordinates of points of the media object. The print head coordinates are coordinates of a position of the print head when ejecting recording material on the media object. The print head coordinates are derived via coordinates of the flat bed surface. By using the coordinates of the flat bed surface more calculations and calibrations are necessary and errors like approximation errors are introduced. This way of working leads to an inaccuracy in determining the print head coordinates.
It is an objective of the present invention to achieve an increase of the accuracy of the determination of the print head coordinates and thus to achieve a better print quality of a print on the media object.